Bicycle helmets have now become ubiquitous for the bicycling activity. In road and urban riding, one specific helmet construction has become the norm: that consisting of the foam inner liner with an outer shell. The inner liner forms the body of the helmet in terms of volume and structural integrity. The inner liner is typically made of a structural foam material such as expanded polystyrene. An outer shell covers the liner and defines the smooth and decorative exposed outer surface of the helmet. The outer shell and liner are most often co-molded. Other components include the attachment system inside the outer shell, by which the helmet is secured to the user's head.
The above-referred configuration is quite convenient in terms of providing suitable head protection, while being lightweight. Moreover, in some instances, numerous vents may be defined in the helmet to allow air circulation and the exhaust of sweat, which is often necessary in warmer riding weather.
Helmets have been shown to be non-optimal in terms of aerodynamics, notably because of the presence of such vents causing additional drag. Accordingly, helmets used in competitions have recently been designed with fewer vents to limit drag losses. For example, Time trial helmets are often with very few vents. However, such helmets may not be as comfortable in warm weather.